ABSTRACTThe Gibson Dome area in the Paradox Basin in southeastern Utah (Figure 1) has been identified as a potential location for a high-level nuclear waste repository on the basis of an adequate thickness of bedded salt formations at desirable depths, suitable topography for surface facilities, few known archaeological sites, less resource potential than otherwise similar areas, and long-term geologic and tectonic stability [1]. The area appears also suitable from a geohydrologic viewpoint, on the basis of data collected and analyses performed to date. The regional geohydrologi information was obtained from three deep boreholes drilled as part of the Department of Energy's exploration program and from other existing sources. The upper, near-surface, geologic formations include both regionally continuous water-bearing formations and locally perched ground waters that discharge into nearby surface streams and into the Colorado River. These are underlain by the Paradox formation, which consists of a series of essentially impermeable salt layers and of nonsalt interbeds with negligible ground water. Below the Paradox salts, the formations of interest with respect to repository safety include regionally continuous water-bearing formations, with the Leadville limestone being the principal water-transmitting unit. Flows in all water-bearing formations are essentially horizontal. If a vertical connection were established through a potential repository, hydraulic gradients indicate that the flow would first be downward from the upper to the lower formations and then laterally, principally in the Leadville formation. Surface discharge of Leadville formation ground water from the Gibson Dome area has not been detected. There are some indications that minor leakage could occur into the Colorado River as close as Cataract Canyon, about 20 to 25 km (10 to 15 miles) from a potential repository location in Davis Canyon, or into the Colorado River in Marble Canyon (Arizona), about 240 km (150 miles) to the southwest. The computed ground-water flow time in the Leadville formation from below Davis Canyon is about 90,000 years for the first 10 km (6 miles) and about 200,000 years to below Cataract Canyon. The ground-water flow time is estimated to exceed 500,000 years to Marble Canyon. Groundwater flow from a repository to these locations is unlikely, however, since water would first have to penetrate the essentially impermeable salt layers before it would reach the Leadville formation.
